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Abstract

This article describes a scheme for security of data in a computer system wherein an encryption/decryption subsystem is placed between the host and asynchronous communications subsystem (ACS) hardware.

Country

United States

Language

English (United States)

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Page 1 of 3

Adding Data Security to Asynchronous Communications

This article describes a scheme for security of data in a computer system
wherein an encryption/decryption subsystem is placed between the host and
asynchronous communications subsystem (ACS) hardware.

There are two ways to protect data on computer systems or between
computer systems. The first method is physical protection. This method controls
physical access to computer systems and data. There are times when denying
physical access to portions of a computer system can be difficult or even
impossible. Data communications frequently take place over public systems
such as telephone lines and microwave transmissions. It is impossible to restrict
access to all portions of the telephone network. Although data transmissions
cannot be prevented from interceptions, a second form of protection for security
can be provided.

Security insures that intercepted data transmissions are unintelligible and
hence of no use to the "thief". Security will be provided in the form of data
encryption.

An encryption "front end" is added to the ACS. An encryption/ decryption
subsystem is placed between the host and the ACS hardware. A block diagram
of this selectable encryption decryption subsystem (SEDS) is shown in the
drawing. SEDS is fully programmable and only operates when enabled. When
disabled, SEDS performs no data manipulation. When enabled and initialized,
SEDS operates transparently to an application. That is, no change in the
algorithms used to handle the ACS is required.

When enabled, SEDS will perform data encryption/decryption. When the host
outputs a clear text data byte, SEDS will encrypt the byte and present it to the
ACS. When the ACS produces a cipher text data byte, SEDS will decrypt it and
present it to the host.

Since SEDS is a subsystem between the ACS and the host, SEDS is
independent of any buffering occurring in the ACS. Thus, in a mixed clear text,
cipher text transmission SEDS can be enabled or disabled at the appropriate
time. If SEDS is between the ACS and the communications line, buffering could
result in SEDS being switched off too late in a transmission. For example, a
cipher text data stream contains an indication to discontinue cipher text mode.
The indication and subsequent data bytes are in the ACS buffer. When the
indication is finally processed, the clear text data bytes have already been
processed by SEDS. This motivates the placement of SEDS between the host
and the ACS.

The drawing illustrates the interconnection of the data cipher processor
(DCP) unit and the communications controller (CC) to the host. Both the DCP
unit and the CC present three-state buses for control and data exchange with the
host. The host data bus is also buffered by a three-state device. Multi-port
control logic (MPCL) is responsible for decoding the host bus signals and
steering data to the appropriate unit. Normal operation of the CC entails sending
various bytes of setup data via its I...